1. Field of the Invention
The present invention relates to a metallic member, on one of whose surfaces a zinc composite coating film and a coating layer are formed and which is thereby subjected to a rust-preventive treatment. Moreover, it relates to a coating composition, which is used for metallic members of this type.
2. Description of the Related Art
As a rust-preventive technique, it has been known conventionally to form a zinc composite coating film on a surface of metallic member, thereby undergoing the metallic member a rust-preventive treatment. Because of the rust-preventive action (or self-sacrificing corrosion preventive action) of zinc that is included in the zinc composite coating film, rust is less likely to occur on metallic members that are subjected to rust-preventive treatments of this kind. Moreover, as disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-146,005, a technique is proposed in order to maintain the rust-preventive action of zinc over a long period of time by forming a coating layer on the zinc composite coating film.
Specifically, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-146,005 discloses such a technique as forming a zinc composite coating film by performing a metallic powder-chromium oxide treatment (or so-called “DACTROTIZED” treatment (Trademark)) to a surface of metallic member and then further forming a coating layer, which is made from an aqueous processing liquid, on the resulting zinc composite coating film. For example, the aqueous processing liquid contains an aqueous resin, and a silica material. The technique disclosed in the publication makes it possible to maintain the rust-preventive action of the zinc composite coating film for a long period of time relatively, because the coating layer can isolate the zinc composite coating film and metallic member from environments. Moreover, the coating layer might be able to deform so as to follow the metallic member's elastic deformations, because it contains a resinous component. Therefore, even when the resultant metallic member is used for hose clamps and springs, it might be possible to shut off the zinc composite coating film and metallic member from the surroundings.
However, in the raw material of the coating layer (hereinafter referred to as a “coating composition”) that is disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-146,005, an aqueous resin and a silica material are dispersed in water. The aqueous resin in such a coating composition disperses in water as emulsion particles, because it does not dissolve in water. To put it differently, the aqueous resin exists as particles with relatively large molecular weights in the coating composition. Accordingly, it is hard to control the viscosity of coating composition by the technique disclosed in the publication, and consequently it has been difficult to make the resulting coating layer thinner.
Moreover, in a coating composition of this type, the aqueous resin, which comprises polymeric emulsion particles, cures to form networks when the coating composition forms a coating layer, in particular when it undergoes a baking treatment. Accordingly, it is difficult to soften the resulting coating layer sufficiently by the technique disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-146,005, and consequently it has been difficult to produce a coating layer which is less likely to come off, or in which cracks are less likely to occur. When a coating layer comes off, it is difficult to maintain the rust-preventive action of zinc composite coating film for a long period of time. Therefore, it might have been difficult to give metallic members, which are subjected to rust-preventive treatments, much better rust-preventive performance.